A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103, which in turn vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The scala tympani forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain. Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104.
In some cases, hearing impairment can be addressed by a cochlear implant (CI) that electrically stimulates auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along an implant electrode. FIG. 1 shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processing stage 111 which implements one of various known signal processing schemes. The processed signal is converted by the external signal processing stage 111 into a digital data format, such as a sequence of data frames, for transmission via external coil 107 into a receiver processor in an implant housing 108. Besides extracting the audio information, the receiver processor in the implant housing 108 may perform additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through wires in an electrode lead 109 to an implanted electrode array 110. Typically, the electrode array 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
Conventionally, the implant housing is placed in a bony bed or flattened area drilled on the skull bone. This is done for various reasons including improved stability and protection when external forces act on the implant housing. Recessing the implant housing in the skull bone also reduces the amount by which the implant housing protrudes out from the bone surface towards the skin and makes the implant bump on the skin less obvious when seen from the outside.
Adults have comparatively thick skull bones which generally allow the drilling of deep implant beds and consequently a good fixation of the device. But the skull bones in children are much thinner and it may be difficult to sufficiently recess the implant housing without drilling all the way through the skull bone down to the outer layer of the cerebral membrane, the dura mater. In many young patients, the implanting surgeon decides to remove all underlying bone in order to obtain an appropriately recessed implant housing.
Removing the bone volume in the implant bed by layer by layer drilling is a time consuming task, and minimizing the implant surgery time is gradually becoming more important, not only to minimizes the costs of surgery, but also to reduce the time that the implant patient is under general anesthetic. Therefore, some surgeons use a faster method of making the housing recess where they only drill down to the dura mater along the contour of the implant and pry out the remaining central bone island. This leaves a well that goes all the way through the skull bone, i.e. a recess of maximum possible depth.
Other tasks that can take considerable amounts of time during implantation surgery include fixation of the implant housing in the bony recess. While specific implant fixation (e.g. tying down with sutures) is strongly recommended by cochlear implant manufacturers, some surgeons prioritize a shorter surgery time over the benefits of direct implant fixation and do not specifically fixate the device. Commonly practiced indirect fixations are achieved by tightly closing the periosteum over the implant housing and suturing the surgical opening in the skin over the implantation site.
Even though such methods provide some fixation of the implant housing, they are likely to leave the device in just a semi-fixated situation, at least initially and depending on how well the housing recess was drilled to fit the implant housing. If a deep enough housing recess has been made, the implant housing may be appropriately immobilized in the lateral direction by the bone surrounding the implant housing. But in the perpendicular up-down directions (away from/toward the center of the head), the fixation will most likely be inadequate, especially if there is no bone underneath the implant housing. Movements in towards the brain may occur, for example, when the patient presses on the implant by hand or rests their head on the implant location. Movements in both directions (up/down) may originate from blood pressure pulsations in the brain that make the cerebral membranes move—since the implant housing rests on these membranes it will likely experience similar types of movements.
In an upward direction away from the center of the head, movement of the implant housing is only limited by the periosteum and the skin if there is no direct implant fixation. The periosteum is a dense connective tissue, so it has a limited ability to elongate when non-permanent forces act on it, but it will remodel over time to relieve any permanent tension that may be present, thus becoming a largely tension-free but tightly fitting cover over the implant housing after some period of time.
In its natural condition the periosteum adheres quite strongly to the skull bone. During the surgical implantation procedure this tissue is intentionally loosened from the underlying skull bone to expose an area where the housing recess can be made and to create a periosteal pocket for the implant coil. Normally the periosteum is loosened over an area significantly larger than the implant housing leaving a fairly lose cover over the device directly after implantation, but over time the loosened periosteum will re-attach to the bone by scar tissue formation.
Young cochlear implant users are likely to be reimplanted several times during their lives, thus necessitating repeated surgery at the same location. Thus it is should be assumed that a gradual destruction of the periosteum and its fixating function in the upward direction occurs. Over time new bone tissue will be generated around the implant housing so that it eventually will be found in a well-fitting implant bed. Implant fixation methods also should allow the device to be easily removed if it, for example, becomes non-functional and needs replacement or if the user desires a technological upgrade to a newer device.
Downward (inward) directed movements of the implant housing are generally resisted only by the bone beneath the device and by any parts of the implant system that are attached to the implant housing (e.g. the coil and electrodes) which lie on top of the bone. In cases where all of the bone down to the dura mater has been removed during implantation, some immobilizing function may be provided by the outer cerebral membrane directly underneath the housing which normally adheres to the inside of the skull bone. In many cases, new bone regrows over time over the cerebral membrane giving a better fixation below the implant housing than from just the membrane alone.
Modern CI's are designed to withstand an increased level of impact energy before they become non-functional to be robust against accidental impacts. However, this may be of secondary importance if the impact energy cannot be directed onto stable structures that can take such impact loads without being damaged. If there is no or little bone beneath the implant housing, then there is an inherent risk of the implant housing being displaced in towards the brain when external forces strike the implant (e.g. from a CI user accidentally falling and hitting the head). This risk goes together with increased risks of hemorrhages and other tissue damages in the area of the brain which can have serious consequences.
In recent years some efforts have been made to develop ways of easy, fast, reliable, and safe fixation of implantable neuro-stimulators such as cochlear implants. Direct and indirect suturing of the implant housing are the most common ways of securing the devices, but more and more surgeons are moving away from direct suturing, either due to time (and cost) concerns or because they believe that this type of fixation is not needed. However, many surgeons do not realize that inappropriate fixation can have a detrimental effect on the long term functionality of the device and so initially saving surgery time could well result in early device failure and the need for an early device replacement surgery.
U.S. Patent Publication 2010-0049318 describes some ways to fix the implant housing to the bone underneath the device. But if there is no or little bone under the implant housing, then some of the described fixation methods are inappropriate. Similar concepts are described in U.S. Patent Publication 2006-0116743 where one or more flanges extend outward from the implant housing for fixation to the tissue. U.S. Patent Publication 2009-0209806 describes a bone conductor transducer that is connected to the skull bone to transmit vibrations using static force methods.